1. Field of the Invention
The present invention relates to a piezoelectric element, such as a three-terminal piezoelectric filter element used, e.g., in an IF (intermediate frequency) filter for an AM radio receiver, or a two-terminal piezoelectric resonator element used, e.g., in a ladder filter or in an oscillator.
2. Description of the Prior Art
An example of a three-terminal piezoelectric filter element 5 is shown in FIGS. 1a-1c, which includes a rectangular piezoelectric plate 1, a ring electrode 2, a center electrode 3 and a ground electrode 4. Ring electrode 2 and center electrode 3 are deposited on one surface of piezoelectric plate, and ground electrode 4 is deposited on the other surface thereof.
When the three-terminal piezoelectric filter elements are employed in an IF filter of an AM radio receiver, two or more filter elements 5 are connected in cascade with a coupling capacitor Co inserted in the data transmission line D, for example as shown in FIG. 2. In FIG. 2, the data transmission line D is connected between two center electrodes, but it can be connected between two ring electrodes, or between center and ring electrodes. The capacitor Co inserted in the data transmission line is provided for reducing the coupling efficiency.
From the viewpoint of minimizing the size of the IF filter, it is preferable to eliminate the coupling capacitor Co which is usually provided externally to the chip element containing other circuit elements. However, when the coupling capacitor Co is eliminated so as to directly connect the center electrodes, the coupling efficiency will not be decreased to the required level. In this case, the coupling efficiency may be reduced by reducing the electrostatic capacitance between the center electrode 3 and ground electrode 4, and also reducing the conversion efficiency between the electric signal and the mechanical movement in the filter element. However, when the center electrode is reduced in size, there arises another problem namely that unwanted spurious mode vibration will be produced at frequency ranges of 3, 5 and 7 times the filtering frequency. Therefore, in the prior art, no IF filter of the above described type, without a coupling capacitor Co, has been practically used.
An example of a two-terminal piezoelectric resonator element 55 is shown in FIGS. 3a-3c, which includes a rectangular piezoelectric plate 57, a first electrode 58 and a second electrode 59. The first electrode 58 and the second electrode 59 are deposited on the opposite surfaces of the piezoelectric plate. The size of the first electrode 58 is smaller than that of the second electrode 59 so as to reduce the electrostatic capacitance. In use, at least one serial connected two-terminal piezoelectric resonator 55 and at least one parallel connected two-terminal piezoelectric resonator 56 are connected in a manner shown in FIG. 4 to define a ladder filter between input terminals 51 and 52 and output terminals 53 and 54.
However, when the first electrode is reduced in size, there arises the same problem as mentioned above, namely that unwanted spurious mode vibration will be produced at frequency ranges of 1.5, 3, 5 and 7 of the resonance frequency.